1. Field of the Invention
This invention relates to high frequency antenna systems, and more particularly to cavity-backed antenna systems.
2. Description of the Prior Art
For many applications of high frequency antenna systems, for example, angle-of-arrival monopulse detection, it is advantageous to use a circularly polarized antenna system having a relatively high gain and whose gain pattern is characterized by symmetry. It would also be advantageous to provide an antenna system sensitive to both senses of circularly polarized radiation, that is, right circular polarization (RCP) and left circular polarization (LCP).
One type of antenna system which has been considered for angle-of-arrival detection applications is the spiral antenna, and in particular quad spiral antennas comprising four spiral conductive elements. While the polarization of this type of antenna system is circular, it suffers several disadvantages, including its relatively low gain as a result of resistive absorbers coupled to the outer ends of its spiral elements and/or placed at the bottom of the cavity and its sensitivity to only one sense of circular polarization.
Antenna feed systems are also known wherein concentric, nested, circular cavity-backed wing dipole structures are employed which have a relatively high efficiency combined with broad (multiple octave) bandwidth. Yet these systems are unsuited to use in angle-of-arrival detection systems because of their limited direction finding capability and nonsymmetric gain pattern.
Typical monopulse systems employing X-Y (horizontal and azimuth) coordinates to define the target have detected the antenna sum signal, and two antenna difference signals (azimuth and horizontal), for a single polarization sense. It has, therefore, been customary to employ three receiver channels, one for each of the sum and difference signals. It would be desirable to minimize the number of required receivers for either one or both polarization senses.
It is, therefore, an object of the present invention to provide an antenna system having dual circulary polarized monopulse capabilities over a very wide (multi-octave) bandwidth.
It is another object of the present invention to provide a high frequency antenna system having an accurate direction finding capability which is independent of polarization of incident radiation with little sacrifice in efficiency or bandwidth.
It is yet another object of the present invention to provide a high frequency antenna system whose sum and difference patterns are substantially symmetrical in space about its boresight.
Another object of the invention is to provide an antenna system of circular polarization which has a relatively high gain.
It is yet another object of the present invention to provide an antenna system of dual circular polarization, adapted to provide sum and difference signals for either sense of polarization and requiring only two receiver channels for each polarization sense.
It is a further object to provide an antenna system adapted to concentrically nested configurations operable in multiple frequency bands, which obtains the advantages of known concentrically nested structures with their high efficiency and broad bandwidth, while at the same time providing the desired symmetry and uniform sensitivity to polarized electromagnetic radiation.